A well known test for the determination of cardiac output involves the injection of a measured amount of cold injectate solution into the right heart proximal to the pulmonary artery in a predetermined time period of short duration, such as, on the order of two seconds. The temperature drop of the blood passing a thermistor positioned in the heart is then sensed and measured. The decrease in blood temperature in a given time resulting from the injectate solution, when integrated by a cardiac output computer, is a measure of the output capacity of the heart in liters per minute. This technique for determining cardiac output is well known and is of considerable importance in diagnosing and treating critically ill patients. The value of the technique of thermodilution cardiac output monitoring is directly related to the accuracy of the process. Many thermodilution cardiac output computers are commercially available for obtaining determinations of cardiac output from a blood temperature drop curve.
The reliability of the technique of thermodilution cardiac output monitoring depends on the accuracy and repeatability of the injection process. At the present time the greatest potential source of error is in the time period for the introduction of the injectate. In order for the output readings to be accurate, repeatable and reliable, the injectate must be delivered to the patient over a short predetermined time period, which time period must be the same for each injection. If the time period of injection varies, the rate of change of blood temperature over a given time will also vary, and the computer output readings will thus be rendered inaccurate and unreliable. Bearing in mind that injection should occur over a time period of approximately two seconds, the time it takes for 10 cc of O'dextrose to be injected manually, it can readily be seen that a variation of as little as a fraction of a second from injection to injection can lead to substantial errors in measurement.
In the present practice of the thermodilution injection technique, a doctor or medical technician manually operates a syringe to deliver the injectate into a catheter placed in the right heart proximal to the pulmonary artery. Manual introduction of the injectate has the potential for significant inaccuracies which in turn, lead to serious errors in the computer output. It is extremely difficult for a medical technician to deliver a steady flow of the injectate repeatably over the same time period, and it is even more difficult for different medical technicians to deliver the full amount of injectate in the identical time period repeatably. Thus, the delivery rate of the injectate usually varies, and the time period is usually somewhat greater or somewhat less than the time period for the previous injection. As a result large fluctuations of cardiac output are routinely observed in a series of determinations done on the same patient by different operators. The most probable cardiac output volume is arrived at by sampling several of the closest readings and rejecting the rest.